Laboratory gradientless reactor

In gradientless reactors the catalytic rate is measured under highly, even if not completely uniform conditions of temperature and concentration. The reason is that, if achieved, the subsequent mathematical analysis and kinetic interpretation will be simpler to perform and the results can be used more reliably. The many ways of approximating gradientless operating conditions in laboratory reactors will be discussed next. 

Many configurations of laboratory reactors have been employed. Rase (Chemical Reactor Design for Proce.s.s Plants, Wiley, 1977) and Shah (Ga.s-Liquid-Solid Reactor Design, McGraw-Hill, 1979) each have about 25 sketches, and Shah s bibliography has 145 items classified into 22 categories of reactor types. Jankowski et al. (Chemlsche Tech-nik, 30, 441 46 [1978]) illustrate 25 different lands of gradientless laboratory reactors for use with solid catalysts. 

The concentration-controlled, gradientless differential circulating reactor is best suited for kinetic measurements. Such modem laboratory reactors are now of major importance. They allow kinetic data to be measured and evaluated practically free of distortion by heat- and mass-transport effects [17]. Depending on the material flow, a distiction is made between reactors with outer and inner circulation. Evaluation of the kinetic measurements is straightforward because the simple algebraic balance equation for a stirred tank reactor (Eq. 13-8) can be applied (prerequisite high recycle ratio R). In practice it is foimd that recycle ratios of R = 10-25 are sufficient to achieve practically ideal stirred tank behavior [8]. 

Figure4.11.1 Gradientless laboratory reactors (a) continuous stirred tank reactor (b) recycle reactor.

Hence we need respective criteria for the design and operation of a laboratory reactor to ensure negligible deviations from the ideal. Subsequently, we repeat these criteria, which were already derived in Sections 4.7, 4.10.6.5, and 4.10.7.2, and specify them for laboratory-scale experiments. In the next subsection, the criteria for ideal plug flow behavior (exclusion of an influence of axial and radial dispersion of mass and heat are covered), and in the subsequent subsection, the criteria for gradientless deal particle behavior (exclusion of an influence of interphase and intraparticle transport of mass and heat) are outlined. 

Different laboratory reactors are used for kinetic studies. For studies of liquid-liquid reactions and homogeneously catalyzed reactions, a batchwise operated stirred tank reactor is frequently used. Tubular reactors loaded with catalyst (fixed bed) are more common for studies of heterogeneously catalyzed gas reactions. The tubular reactor displays a simple design and is easy to operate. A simultaneous integral and differential mode of operation can be achieved by a tap reactor for measuring concentration and temperatures at defined axial positions. Gradientless operation with respect to temperature and concentration can be obtained by an external or internal recycle. 

The most appropriate laboratory reactor for detailed kinetic investigations is the continuously operated, gradientless recycle reactor. A large number of different constructions is described in the literature /44/. We have developed and successfully used for many years for heterogeneously catalysed vapor phase reactions such a reactor with internal recirculation (Fig. 21) It can be operated up to 800 K and 50 bars (catalyst volume 10 cm ). 

Fig. 21 Construction details of a gradientless laboratory reactor with internal recirculation (29 catalyst basket, 32 turbine, 41 motor)...

While transport effects may be eliminated in laboratory reactors, and experiments have shown that certain reactions oscillate under what may be considered isothermal and gradientless conditions, the Langmuir-Hinshelwood mechanism by itself with conventional mass-action kinetics does not give a satisfactory description of them. A number of "extra" features have been added in modeling studies reported in the literature. Among them are an activation energy which depends on the concentration of adsorbed species in one or more of the reaction steps [37, 52 - 54], transition between active and inactive forms of an adsorbed component [7, 17, 55, 56], and periodic switching of the reaction mechanism [16, 18, 40, 57]. 

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